Can't tune maxsize of connection pool used

I get a *lot* of log messages of the form:

```
WARNING:urllib3.connectionpool:Connection pool is full, discarding connection: 10.0.0.1
```

Setting `maxsize` of `HTTPConnectionPool` (or equivalent object) to something higher would help with this, and with connection re-use help performance too (I am hoping!). I can't seem to find a way to set this though!